Abstract : This report is one in a series of Software Engineering Institute (SEI) case studies in software architecture. It describes structural modeling, a technique for creating software architectures based on a small set of design elements called structural types. Structural modeling resulted from the efforts of the Air Force Aeronautical Systems Command (ASC/YW) and has been used by Air Force contractors since the late 1980s to design large-scale, high-fidelity aircrew trainer simulation software. This report examines the changes, resulting from the use of structural modeling, to the trainer's software architecture and to the development methods used.